Transmit coil frequency response correction for magnetic resonance imaging

ABSTRACT

Methods for correcting a non-uniform power response of a radiofrequency (“RF”) transmit coil used in magnetic resonance imaging (“MRI”) are described. Transmit power response data for an RF transmit coil are processed to compute RF amplitude scaling factors for the RF transmit coil as a function of transmit frequency offset. The RF amplitude scaling factors can be used to correct transmitted RF power, and thus flip angle, to be more uniform over a range of transmit frequency offsets, as may be encountered when imaging with lower field MRI systems or MRI systems with high strength or asymmetric gradients.

BACKGROUND

Radiofrequency (“RF”) transmit coils used in magnetic resonance imaging(“MRI”) are normally designed to be as efficient as possible, typicallycharacterized by the “Q” of the coil. In particular, RF transmit coilsare designed to be efficient at converting power that is provided to thecoil into the transmitted RF power. In addition, RF transmit coils needto be tuned so that their peak efficiency occurs close to the expectedresonance frequency of the resonant species that is to be investigated.For example, when imaging protons (i.e., the most commonly imagedresonant species), the RF transmit coil needs to be tuned toapproximately 42.57 MHz per Tesla of the magnetic field strength of theMRI system's main magnetic field, B₀. However, the width of thisresonant tuning also tends to scale with magnetic field strength, sothat the RF transmit coil is efficient over a wider range of frequenciesat higher magnetic field strengths, but is efficient over a smallerrange of frequencies at lower magnetic field strengths.

In addition, when performing slice-selective excitations in normal MRIapplications, when an off-center slice needs to be excited thetransmitted RF pulse may need to be applied with an offset frequencyfrom the nominal resonance frequency of the target resonant species. Inthese situations, the transmit frequency is offset based on the distanceof the slice from center, the transmit bandwidth of the RF pulse (whichis most commonly influenced by the duration of the RF pulse), and thestrength of the applied slice-selective gradient. The net result is thatfor thin, off-center slices excited with short RF pulses, the RFexcitation pulse may need to be transmitted at a large offset frequency.

Some MRI systems implement an asymmetric gradient design, in which thegradient does not produce a net zero field that coincides spatially withthe magnet isocenter. In these systems, the asymmetric gradient actslike a nominal slice offset in the corresponding direction, which canrequire a large offset frequency even for slices excited at isocenterbecause of the additional non-zero gradient in the asymmetric gradientdirection.

SUMMARY OF THE DISCLOSURE

The present disclosure addresses the aforementioned challenges byproviding a method for producing an image with a magnetic resonanceimaging (“MRI”). The method includes providing radiofrequency (“RF”)amplitude scaling data to the MRI system. The RF amplitude scaling datacontains RF amplitude scaling factors that define an RF amplitudescaling to apply for a particular transmit frequency. Data are acquiredfrom a subject with the MRI system by performing a magnetic resonancepulse sequence that includes generating at least one RF pulse with an RFtransmit coil. An amplitude of the at least one RF pulse is scaled usingthe provided RF amplitude scaling data to correct for a non-uniformtransmit power response of the RF transmit coil. An image of the subjectis reconstructed from the acquired data.

It is another aspect of the present disclosure to provide a method forcomputing RF scaling factors for correcting a non-uniform transmit powerresponse of an RF transmit coil used in an MRI system. Transmit powerresponse data for the RF transmit coil are provided to a computersystem. The transmit power response data define an amount by which RFpower transmitted by the RF transmit coil changes as a function oftransmit frequency. RF amplitude scaling factors are computed with thecomputer system by computing a mathematical inverse of the transmitpower response data. The RF amplitude scaling factors are stored in thecomputer system as RF amplitude scaling data for later use by the MRIsystem.

It is another aspect of the present disclosure to provide a method forproducing RF transmit pulse waveforms for use with an MRI system thatincludes an RF transmit coil. Transmit power response data for the RFtransmit coil are provided to a computer system. The transmit powerresponse data defines an amount by which RF power transmitted by the RFtransmit coil changes as a function of transmit frequency. RF amplitudescaling factors are computed with the computer system based at least inpart on the transmit power response data. An RF transmit pulse waveformis generated based at least in part on a pulse duration selected withthe computer system, a transmit frequency selected with the computersystem, and an amplitude selected with the computer system and scaledusing the RF amplitude scaling factor associated with the selectedtransmit frequency.

The foregoing and other aspects and advantages of the present disclosurewill appear from the following description. In the description,reference is made to the accompanying drawings that form a part hereof,and in which there is shown by way of illustration a preferredembodiment. This embodiment does not necessarily represent the fullscope of the invention, however, and reference is therefore made to theclaims and herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart setting forth the general steps of an examplemethod for computing and storing radiofrequency (“RF”) amplitude scalingfactors computed from transmit power response data for an RF transmitcoil.

FIG. 2 shows an example of transmit power response data for an RFtransmit coil.

FIG. 3 shows an example of the inverse of the transmit power responsedata of FIG. 2.

FIG. 4 is a flowchart setting forth the steps of an example method forcorrecting an RF transmit coil for a non-uniform transmit power responseusing RF amplitude scaling factors, such as those computed using thegeneral method of FIG. 1.

FIG. 5 is a block diagram of an example computer system that canimplement the methods described in the present disclosure.

FIG. 6 is a block diagram of an example magnetic resonance imaging(“MRI”) system that can implement the methods described in the presentdisclosure.

DETAILED DESCRIPTION

Described here are methods for correcting a non-uniform power responseof a radiofrequency (“RF”) transmit coil used in magnetic resonanceimaging (“MRI”). As described above, when imaging a subject using an MRIsystem with a low main magnetic field strength (e.g., less than 1.5 T),an asymmetric gradient design, or both, the transmit frequencies used toexcite off-center slices may be offset from the tuned resonance of theRF transmit coil by a degree sufficient to result in significanttransmit power reduction. The methods described in the presentdisclosure address this problem with variable transmit power (and thusvariable flip angle) as a function of transmit offset frequency.

This problem described above can often occur in MRI systems with lowermain magnetic field strengths (e.g., less than 1.5 T), high gradientstrengths, asymmetric gradients, large transmit bandwidths, andcombinations thereof.

Referring now to FIG. 1, a flowchart is illustrated as setting forth thesteps of an example method for correcting, or otherwise compensating,for a non-uniform power response of an RF transmit coil.

The method includes providing to a computer system transmit powerresponse data for an RF transmit coil, as indicated at step 102.Providing the transmit power response data can include retrievingpreviously measured, or otherwise characterized, transmit power responsedata from a memory or data storage, or can include measuring orotherwise characterizing the transmit power response with the computersystem. In general, the transmit power response of the RF transmit coilis measured, or otherwise characterized, as a function of transmitfrequency. The transmit power response data thus provides the amount bywhich the transmitted RF power changes as a function of transmitfrequency. An example of transmit power response data is shown in FIG.2.

The transmit power response data can be measured, for example, bymeasuring the peak RF power for a given RF transmit coil at a particularoffset frequency. A series of such measurements can be made whilevarying to offset frequency to provide a measurement of the transmitpower response as a function of transmit frequency. In some instances, adifferent set of transmit power response data can be measured fordifferent main magnetic field strength (B₀) values. Although changingthe strength of the main magnetic field may not change the transmitpower response of the RF transmit coil, the range of relevant transmitfrequencies over which the transmit power response is measured will bedifferent depending on the main magnetic field strength. Thus, differenttransmit power response data can be measured over different transmitfrequency ranges depending on the main magnetic field strength that willbe used. This approach is particularly useful for MRI systems that areable to rapidly ramp their main magnetic field strength, such as the MRIsystem described in co-pending U.S. patent application Ser. No.15/128,881, which is herein incorporated by reference in its entirety.

Referring still to FIG. 1, the method for correcting, or otherwisecompensating, for a non-uniform power response of an RF transmit coilproceeds by computing RF amplitude scaling factors based at least inpart on the provided transmit power response, as indicated at step 104.As one example, the RF amplitude scaling factors can be computed bycomputing the inverse of the transmit power response. An example of RFamplitude scaling factors is shown in FIG. 3.

The RF amplitude scaling factors are then stored for later use as RFamplitude scaling data, as indicated at step 106. These RF amplitudescaling data can be used to adjust the implementation of RF pulses(e.g., RF excitation pulses) using in an MRI pulse sequence to providefor more uniform RF transmit power. It will be appreciated that RFamplitude scaling data can be generated and stored for multipledifferent RF transmit coils, such that the appropriate data can beretrieved when a particular RF transmit coil will be used.

Referring now to FIG. 4, a flowchart is illustrated as setting forth thesteps of an example method for operating an RF transmit coil in an MRIsystem in a manner that results in more uniform RF transmit power. Themethod includes providing to the MRI system, RF amplitude scaling data,as indicated at step 402. Providing the RF amplitude scaling data caninclude retrieving previously generated RF amplitude scaling data (e.g.,RF amplitude scaling data generated using the methods described in thepresent disclosure) from a memory or other data storage, or bygenerating such data using the methods described in the presentdisclosure.

The provided RF amplitude scaling data are then communicated to the MRIsystem for use during a pulse sequence, as indicated at step 404. Forinstance, the RF amplitude scaling data can be communicated to an RFsystem or subsystem of the MRI system. The MRI system then uses the RFamplitude scaling factors stored in the RF amplitude scaling data toadjust the amplitude of RF pulses (e.g., RF excitation pulses) in thepulse sequence, as indicated at step 406. For instance, the RF system orsubsystem uses the RF amplitude scaling data to request higher RF powerfor pulses at higher offset frequencies, so as to make all RF pulseshave the same flip angle, regardless of offset frequency. This can beachieved by scaling the requested RF pulse amplitude by the appropriateRF amplitude scaling factor based on the desired transmit frequencyoffset.

If the maximum transmit coil power is exceeded after this correction(e.g., whether a maximum threshold value is exceeded), as determined atdecision block 408, then the RF pulse duration for each pulse thatexceeds the maximum coil power can be extended until peak power limitsare satisfied, as indicated at step 410. In some implementations, if themaximum transmit coil power is exceeded by one or more RF pulse, thenthe RF pulse duration of all of the RF pulses can also be extended. As aresult of extending the RF pulse duration, the required offset frequencyfor the RF pulse is reduced, which in turn reduces the amount to scalethe transmit power. The adjusted RF pulse waveforms (e.g., the scaledamplitudes, extended pulse durations) are then used by the MRI system togenerate the desired RF fields during a pulse sequence to acquiremagnetic resonance data from a subject, as indicated at step 412. One ormore images of the subject can then be reconstructed from the acquireddata, as indicated at step 414. The reconstructed images can bedisplayed to a user or stored for additional processing or later use.

Referring now to FIG. 5, a block diagram of an example computer system500 that can perform the methods described in the present disclosure isshown. The computer system 500 is generally implemented with a hardwareprocessor 504 and a memory 506.

The computer system 500 includes an input 502, at least one hardwareprocessor 504, a memory 506, and an output 508. The computer system 500may be implemented, in some examples, by a workstation, a notebookcomputer, a tablet device, a mobile device, a multimedia device, anetwork server, a mainframe, or any other general-purpose orapplication-specific computing device. The computer system 500 mayoperate autonomously or semi-autonomously, or may read executablesoftware instructions from the memory 506 or a computer-readable medium(e.g., a hard drive, a CD-ROM, flash memory), or may receiveinstructions via the input 502 from a user, or any another sourcelogically connected to a computer or device, such as another networkedcomputer or server. In general, the computer system 500 is programmed orotherwise configured to implement the methods and algorithms describedabove.

The input 502 may take any suitable shape or form, as desired, foroperation of the computer system 500, including the ability forselecting, entering, or otherwise specifying parameters consistent withperforming tasks, processing data, or operating the computer system 500.In some aspects, the input 502 may be configured to receive data, suchas transmit power response data for an RF transmit coil. Such data maybe processed as described above to compute RF amplitude scaling factorsfor use in correcting a non-uniform transmit power response of the RFtransmit coil. In addition, the input 502 may also be configured toreceive any other data or information considered useful for computingsuch RF amplitude scaling factors, or for generating or correcting RFtransmit pulse waveforms using the RF amplitude scaling factors.

Among the processing tasks for operating the computer system 500, the atleast one hardware processor 504 may also be configured to carry out anynumber of post-processing steps on data received by way of the input502.

The memory 506 may contain software 510 and data 512, such as transmitpower response data for an RF transmit coil, and may be configured forstorage and retrieval of processed information, instructions, and datato be processed by the at least one hardware processor 504. In someaspects, the software 510 may contain instructions directed to computingRF amplitude scaling factors. The memory 506 may also contain data 512in the form of RF amplitude scaling factors that have been computed fora particular RF transmit coil.

In addition, the output 508 may take any shape or form, as desired, andmay be configured for displaying, in addition to other desiredinformation, reconstructed signals or images.

Referring particularly now to FIG. 6, an example of an MRI system 600that can implement the methods described here is illustrated. The MRIsystem 600 includes a magnet assembly 602 that generates a main magneticfield, B₀, which may also be referred to as a polarizing magnetic field.The MRI system 600 also includes a gradient coil assembly 604 containingone or more gradient coils, which is controlled by a gradient system606, and a radiofrequency (“RF”) coil assembly 608 containing one ormore RF coils, which is controlled by an RF system 610.

The RF coil assembly 608 can include one or more RF coils that areenclosed within a housing 612 of the MRI system 600, or can include oneor more RF coils that are physically separate from the housing 612, suchas local RF coils that can be interchangeably positioned within the boreof the MRI system 600. Similarly, the gradient coil assembly 604 caninclude one more gradient coils that are enclosed within the housing 612of the MRI system 600, or can include one or more gradient coils thatare physically separate from the housing 612 and that can beinterchangeably positioned within the bore of the MRI system 600. Thehousing 612 may be sized to receive a subject's body, or sized toreceive only a portion thereof, such as a subject's head.

The magnet assembly 602 generally includes a superconducting magnet thatis formed as one or more magnet coils made with superconducting wire,high temperature superconducting (“HTS”) wire, or the like. The one ormore magnet coils can be arranged as a solenoid, a single-sided magnet,a dipole array, or other suitable configuration. The superconductingmagnet can be cooled using a liquid or gaseous cryogen, as is known inthe art, or can be cooled using a cryogen-free arrangement. In thelatter example, the superconducting magnet can be cooled using amechanical cryocooler, such as a Gifford-McMahon or pulse tubecryocooler. In some other configurations, the magnet assembly 602 caninclude one or more electromagnets, resistive magnets, or permanentmagnets. For example, the magnet assembly 602 could include a Halbacharray of permanent magnets.

In some configurations, the magnet assembly 602 includes asuperconducting magnet that can be rapidly ramped from a first magneticfield strength to a second magnetic field strength. In these instances,the MRI system 600 can include a magnet controller 614 that controls theramping of the main magnetic field, B₀. An example of such a controlleris described in co-pending U.S. patent application Ser. No. 15/128,881,which is herein incorporated by reference in its entirety.

As will be described, the RF coil assembly 608 generates one or more RFpulses that rotate magnetization of one or more resonant species in asubject or object positioned in the main magnetic field, B₀, generatedby the magnet assembly 602. In response to the one or more transmittedRF pulses, magnetic resonance signals are generated, which are detectedto form an image of the subject or object. The gradient coil assembly604 generates magnetic field gradients for spatially encoding themagnetic resonance signals. Collectively, the one or more RF pulses andthe one or more magnetic field gradients define a magnetic resonancepulse sequence.

In some configurations, the MRI system 600 can also include a shim coilassembly 616. The shim coil assembly 616 can include passive shims,active shims, or combinations thereof. Active shims can include activeshim coils that generate magnetic fields in order to shim, or reduceinhomogeneities, in the main magnetic field, B₀, generated by the magnetassembly 602. In some configurations, the active shim coils arecontrolled by an active shim controller 618. The active shim coils mayinclude adaptive shim coils, such as those described in U.S. Pat. No.9,523,751, which is herein incorporated by reference in its entirety.

The MRI system 600 includes an operator workstation 620 that may includea display 622, one or more input devices 624 (e.g., a keyboard, amouse), and a processor 626. The processor 626 may include acommercially available programmable machine running a commerciallyavailable operating system. The operator workstation 620 provides anoperator interface that facilitates entering scan parameters into theMRI system 600. The operator workstation 620 may be coupled to differentservers, including, for example, a pulse sequence server 628, a dataacquisition server 630, a data processing server 632, and a data storeserver 634. The operator workstation 620, the pulse sequence server 628,the data acquisition server 630, the data processing server 632, and thedata store server 634 may be connected via a communication system 636,which may include wired or wireless network connections.

The pulse sequence server 628 functions in response to instructionsprovided by the operator workstation 620 to operate the gradient system606 and the RF system 610. Gradient waveforms for performing aprescribed scan are produced and applied to the gradient system 606,which then excites gradient coils in the gradient coil assembly 604 toproduce the magnetic field gradients (e.g., G_(x), G_(y), and G_(z)gradients) that are used for spatially encoding magnetic resonancesignals.

RF waveforms are applied by the RF system 610 to the RF coil assembly608 to generate one or more RF pulses in accordance with a prescribedmagnetic resonance pulse sequence. Magnetic resonance signals that aregenerated in response to the one or more transmitted RF pulses aredetected by the RF coil assembly 608 and received by the RF system 610.The detected magnetic resonance signals may be amplified, demodulated,filtered, and digitized under direction of commands produced by thepulse sequence server 628.

The RF system 610 includes an RF transmitter for producing a widevariety of RF pulses used in magnetic resonance pulse sequences. The RFtransmitter may include a single transmit channel, or may includemultiple transmit channels each controlling a different RF transmitcoil. The RF transmitter is responsive to the prescribed scan anddirection from the pulse sequence server 628 to produce RF pulses of thedesired frequency, phase, and pulse amplitude waveform. The generated RFpulses may be applied to the RF coil assembly 608, which as describedabove may include one or more RF coils enclosed in the housing 612 ofthe MRI system 600 (e.g., a body coil), or one or more RF coils that arephysically separate from the housing 612 (e.g., local coils or coilarrays).

The RF system 610 also includes one or more RF receiver channels. An RFreceiver channel includes an RF preamplifier that amplifies the magneticresonance signal received by the RF coil in the RF coil assembly 608 towhich the receiver channel is connected, and a detector that detects anddigitizes the I and Q quadrature components of the received magneticresonance signal. The magnitude of the received magnetic resonancesignal may, therefore, be determined at a sampled point by the squareroot of the sum of the squares of the I and Q components:M=√{square root over (I ² +Q ²)}  (1);

and the phase of the received magnetic resonance signal may also bedetermined according to the following relationship:

$\begin{matrix}{\varphi = {{\tan^{- 1}( \frac{Q}{I} )}.}} & (2)\end{matrix}$

The pulse sequence server 628 may also connect to a magnet controller614 that can be operated to ramp the main magnetic field, B₀, generatedby the magnet assembly 602 from a first magnetic field strength to asecond magnetic field strength. As one non-limiting example, the firstmagnetic field strength can be 0 T and the second magnetic fieldstrength can be 0.5 T.

When the MRI system 600 includes a shim assembly 616 having one or moreactive shim coils, the pulse sequence server 628 can also connect to anactive shim controller 618 to apply shim coil waveforms for generatingmagnetic fields to shim the main magnetic field, B₀, generated by themagnet assembly 602. In some example, the active shim coils can includeadaptive shim coils. Adaptive shim coils can be used to shim the mainmagnetic field, B₀, and also to generate magnetic fields that can beused for spatially encoding magnetic resonance signals. In suchinstances, the pulse sequence server 628 can provide waveforms to theactive shim controller 618 in order to generate such magnetic fields inaccordance with a magnetic resonance pulse sequence.

The pulse sequence server 628 may also connect to a scan room interface638 that can receive signals from various sensors associated with thecondition of the subject or object being imaged, the magnet assembly602, the gradient coil assembly 604, the RF coil assembly 608, the shimassembly 616, or combinations thereof. In one example, the scan roominterface 638 can include one or more electrical circuits forinterfacing the pulse sequence server 628 with such sensors. Through thescan room interface 638, a patient positioning system 640 can receivecommands to move the subject or object being imaged to desiredpositiApplication-ons during the scan, such as by controlling theposition of a patient table.

The pulse sequence server 628 may also receive physiological data from aphysiological acquisition controller 642 via the scan room interface638. By way of example, the physiological acquisition controller 642 mayreceive signals from a number of different sensors connected to thesubject, including electrocardiograph (“ECG”) signals from electrodes,respiratory signals from a respiratory bellows or other respiratorymonitoring devices, and so on. These signals may be used by the pulsesequence server 628 to synchronize, or “gate,” the performance of thescan with the subject's heart beat or respiration.

Digitized magnetic resonance signal samples produced by the RF system610 are received by the data acquisition server 630 as magneticresonance data, which may include k-space data. In some scans, the dataacquisition server 630 passes the acquired magnetic resonance data tothe data processing server 632. In scans that implement informationderived from the acquired magnetic resonance data to control furtherperformance of the scan, the data acquisition server 630 may beprogrammed to produce such information and to convey it to the pulsesequence server 628. For example, during pre-scans, magnetic resonancedata may be acquired and used to calibrate the pulse sequence performedby the pulse sequence server 628. As another example, navigator signalsmay be acquired and used to adjust the operating parameters of the RFsystem 610 or the gradient system 606, or to control the view order inwhich k-space is sampled.

The data processing server 632 receives magnetic resonance data from thedata acquisition server 630 and processes the magnetic resonance data inaccordance with instructions provided by the operator workstation 620.Such processing may include, for example, reconstructing two-dimensionalor three-dimensional images by performing a Fourier transformation ofraw k-space data, performing other image reconstruction algorithms(e.g., iterative reconstruction algorithms), applying filters to rawk-space data or to reconstructed images, and so on.

Images reconstructed by the data processing server 632 can be conveyedback to the operator workstation 620 for storage. Real-time images maybe stored in a data base memory cache, from which they may be output tooperator display 622 or to a separate display 646. Batch mode images orselected real-time images may also be stored in a data storage 648,which may be a host database containing a disc storage. When such imageshave been reconstructed and transferred to storage, the data processingserver 632 may notify the data store server 634 on the operatorworkstation 620. The operator workstation 620 may be used by an operatorto archive the images, produce films, or send the images via a networkto other facilities.

The MRI system 600 may also include one or more networked workstations650. For example, a networked workstation 650 may include a display 652,one or more input devices 654 (e.g., a keyboard, a mouse), and aprocessor 656. The networked workstation 650 may be located within thesame facility as the operator workstation 620, or in a differentfacility, such as a different healthcare institution or clinic.

The networked workstation 650 may gain remote access to the dataprocessing server 632 or data store server 634 via the communicationsystem 636. Accordingly, multiple networked workstations 650 may haveaccess to the data processing server 632 and the data store server 634.In this manner, magnetic resonance data, reconstructed images, or otherdata may be exchanged between the data processing server 632 or the datastore server 634 and the networked workstations 650, such that the dataor images may be remotely processed by a networked workstation 650.

The present disclosure has described one or more preferred embodiments,and it should be appreciated that many equivalents, alternatives,variations, and modifications, aside from those expressly stated, arepossible and within the scope of the invention.

The invention claimed is:
 1. A method for producing an image with amagnetic resonance imaging (MRI) system, the steps of the methodcomprising: (a) providing RF amplitude scaling data to the MM system,the RF amplitude scaling data containing RF amplitude scaling factorsthat define an RF amplitude scaling to apply for a particular transmitfrequency; (b) acquiring data from a subject with the MRI system byperforming a magnetic resonance pulse sequence that includes generatingat least one RF pulse with an RF transmit coil, wherein an amplitude ofthe at least one RF pulse is scaled using the provided RF amplitudescaling data to correct for a non-uniform transmit power response of theRF transmit coil; (c) reconstructing an image of the subject from theacquired data.
 2. The method as recited in claim 1, wherein providingthe RF amplitude scaling data to the MM system comprises retrievingpreviously computed RF amplitude scaling data from a data storage. 3.The method as recited in claim 2, wherein the data storage comprises amemory.
 4. The method as recited in claim 1, wherein providing the RFamplitude scaling data to the MM system comprises providing transmitpower response data for the RF transmit coil to the MM system andcomputing the RF amplitude scaling data as RF amplitude scaling factorsfrom the transmit power response data, wherein the transmit powerresponse data defines an amount by which RF power transmitted by the RFtransmit coil changes as a function of transmit frequency.
 5. The methodas recited in claim 4, wherein the RF amplitude scaling factors arecomputed by computing a mathematical inverse of the transmit powerresponse data.
 6. The method as recited in claim 4, wherein providingthe transmit power response data to the MM system comprises measuring apeak RF power for the RF transmit coil over a range of transmitfrequencies.
 7. The method as recited in claim 1, wherein a duration ofthe at least one RF pulse is increased when the amplitude of the atleast one RF pulse as scaled by the RF amplitude scaling data results ina peak RF power that exceeds a maximum threshold value.
 8. The method asrecited in claim 7, wherein the amplitude of the at least one RF pulseis scaled by a different RF scaling factor in the RF amplitude scalingdata based on the increased duration of the at least one RF pulse.
 9. Amethod for computing radiofrequency (RF) scaling factors for correctinga non-uniform transmit power response of an RF transmit coil used in amagnetic resonance imaging (MM) system, the steps of the methodcomprising: (a) providing transmit power response data for the RFtransmit coil to a computer system, wherein the transmit power responsedata define an amount by which RF power transmitted by the RF transmitcoil changes as a function of transmit frequency; (b) computing RFamplitude scaling factors with the computer system by computing amathematical inverse of the transmit power response data; and (c)storing the RF amplitude scaling factors in the computer system as RFamplitude scaling data for later use by the Mill system.
 10. The methodas recited in claim 9, wherein providing the transmit power responsedata to the computer system comprises retrieving previously measuredtransmit power response data from a data storage.
 11. The method asrecited in claim 10, wherein the data storage comprises a memory. 12.The method as recited in claim 9, wherein providing the transmit powerresponse data to the computer system comprises measuring a peak RF powerfor the RF transmit coil over a range of transmit frequencies.
 13. Themethod as recited in claim 12, wherein providing the transmit powerresponse data to the computer system comprises measuring the peak RFpower for the RF transmit coil over a range of transmit frequencies ateach of a plurality of different main magnetic field strengths.
 14. Amethod for producing radiofrequency (RF) transmit pulse waveforms foruse with a magnetic resonance imaging (MM) system that includes an RFtransmit coil, the steps of the method comprising: (a) providingtransmit power response data for the RF transmit coil to a computersystem, wherein the transmit power response data defines an amount bywhich RF power transmitted by the RF transmit coil changes as a functionof transmit frequency; (b) computing RF amplitude scaling factors withthe computer system based at least in part on the transmit powerresponse data; and (c) generating an RF transmit pulse waveform based atleast in part on: a pulse duration selected with the computer system; atransmit frequency selected with the computer system; an amplitudeselected with the computer system and scaled using the RF amplitudescaling factor associated with the selected transmit frequency.
 15. Themethod as recited in claim 14, wherein the pulse duration selected bythe computer system is increased when the scaled amplitude results in apeak RF power that exceeds a threshold value.
 16. The method as recitedin claim 15, wherein the increased pulse duration results in selecting adifferent transmit frequency such that the amplitude is scaled by adifferent RF amplitude scaling factor that results in a lower peak RFpower.